The invention relates to a new method of design and construction of linear tensioned membrane solar reflectors for solar parabolic trough concentrators, solar linear reflectors, and linear heliostats for solar Fresnel reflecting systems, in particular those that utilize thin flexible films for the membrane substrate.
Linear tensioned membrane reflectors have many advantages over more traditional designs incorporating ridged frame structures. They are relatively light and easy to assemble. In part because of the light weight, multiple reflectors can be mounted on a single frame structure which can be balanced on pillow block bearings allowing for tilting adjustments to be made with minimal energy expended.
Trough-shaped linear tensioned membrane reflectors, such as those shown in U.S. Pat. No. 4,293,192, issued Oct. 6, 1981, to Allen I. Bronstein and U.S. Pat. No. 4,510,923, issued Apr. 16, 1985 to Allen I. Bronstein, usually comprises a frame structure with parallel-facing identical end form members, each describing the desired cross-sectional shape of the reflector. A membrane of highly reflecting material, such as metalized reflective plastic film, is wrapped tightly around the edges of the form members and the membrane. The membrane is then placed under 1000 to 7000 pounds per square inch (PSI) of tension in one direction, usually by moving one of the end form members away from the other.
However, linear tensioned membrane reflector technology presents certain problems that do not exist for linear solar reflector technologies constructed with a rigid structural frame structures, especially when the device utilizes certain materials or laminates, such as plastic films, as the membrane's substrate. For example, Mylar (Biaxially-oriented polyethylene terephthalate boPET polyester film) is a dimensionally stable material that reacts in undesirable ways when the film is placed under compression. A typical means of mounting the membrane is to adhere it to the underside of a metal strap with a structural adhesive, such as epoxy. The strap is then wrapped around the end form and clamped in place. However, as the strap is bent around the end form the strap's inward facing surface and the membrane are placed in compression, wrinkles are produced; they are then crushed and locked in place as the strap is tightened on the end form. These distortions in the film are magnified by the film and transmitted into the membrane as large longitudinal wrinkles and ripples that span across the entire membrane's surface, distorting its shape.
It is an objective of this invention to reduce the wrinkles and other shape distortions that may occur when thin films are used as a membrane substrate in tensioned membrane solar reflectors.